For many patients with hearing impairment, there are several types of implantable hearing devices, such as middle and inner ear implants, that can restore a sense of partial or full hearing. For example, cochlear implants can restore some sense of hearing by direct electrical stimulation of the neural tissue of the cochlea. The cochlear implant typically includes an electrode carrier having an electrode lead and an electrode array, which is threaded into the cochlea. The electrode array usually includes multiple electrodes on its surface that electrically stimulate auditory nerve tissue with small currents delivered by the electrodes distributed along the electrode array. These electrodes are typically located toward the end of the electrode carrier and are in electrical communication with an electronics module that produces an electrical stimulation signal for the implanted electrodes to stimulate the cochlea. In another example, a conventional hearing aid may be used to provide acoustic stimulation to the auditory system in the form of amplified sound when the impairment is related to the operation of the middle ear. In addition, groups of auditory nerve axons may be stimulated with electrodes placed within the modiolus, or auditory structures in the brain may be stimulated with electrodes placed on or within the structures, for example, on or within the cochlear nucleus.
With many implantable hearing devices, it is desirable to be able to deliver fluids, such as a drug solution, locally with the hearing device in order to treat disorders effectively or to prevent diseases, inflammation or apoptosis after implantation of the hearing device or even restore hearing (e.g., stem cells, nerve growth factors). Therefore, long term or intermittent fluid delivery capability is desired in these types of devices. Extremely low flow rates, however, are required for very small compartments, such as the inner ear with a total volume of about 70 μl, or a part of the inner ear, such as the scala tympani with a volume of about 30 μl. Currently, available pumps which are implantable are generally designed for implantation into much larger compartments, such as the abdominal region of the human body. Fluid delivery rates that are suitable for delivery into larger fluid spaces such as the circulation system (e.g., which consist of about 5-7 l of blood on average) or intrathecal (e.g., into the cerebrospinal fluid, which typically consists of more than 100 ml) are not suitable for smaller spaces. In small compartments, the fluid delivery should take place at flow rates that cause no substantial pressure increase, or only minimal increased pressure, within the compartment. Pumps with lower flow rates, however, are typically not suitable for implantation.